<p>
  A statement used to filter down the selected time series.
  Each individual filtering field (<code>filter</code>, <code>key</code>, <code>tags</code>, and <code>hasTags</code>) may be empty.
  At least one must be specified to make up a valid filter.
</p>
<p>
  See <a href="{{ '/docs/query_language#filters | relative_url }}">filtering</a> for details on how to build a filter.
</p>
